Under current circumstances in which environmental issues caused by waste plastics have been focused recently, biodegradable plastics which are degraded after use into water and carbon dioxide by the action of a microorganism have drawn attention.
Conventionally, various investigations have been conducted on biodegradable foamed particles and molded products thereof. For example, foamed particles obtained by crosslinking using organic peroxide or the like in order to improve foamability of the foamed products produced using a biodegradable aliphatic polyester resin obtained by synthesis with a raw material derived from petroleum, and a molded product thereof (for example, see Japanese Unexamined Patent Application Publication No. Hei 10-324766, No. 2001-106821 and No. 2004-10798); aliphatic polyester foamable particles characterized by having a certain melt viscosity through increasing the molecular weight using diisocyanate as a linking agent (for example, see Japanese Unexamined Patent Application Publication No. Hei 6-248106); and polylactic acid foamed particles produced by using a polyisocyanate, a polyhydric alcohol, a polycarboxylic acid or the like as a crosslinking agent, and a molded product thereof (for example, see pamphlet of International Publication No. 99/21915, Japanese Unexamined Patent Application Publication No. 2000-169546, Japanese Unexamined Patent Application Publication No. 2000-17039, No. 2000-230029, No. 2001-98044, No. 2002-327037, No. 2003-253107, No. 2004-107430, No. 2004-107505 and No. 2004-149649) are included.
However, any of the aforementioned biodegradable aliphatic polyester resins is obtained by synthesis with a raw material derived from petroleum, and is inferior in hydrolysis resistance. Therefore, the foamed and molded product thereof also involves problems of the physical properties after production, i.e., brittleness and the like gained due to lowering of the molecular weight. Furthermore, there are restrictions on degradation conditions in disposal because it is not anaerobically degraded. In order to solve these problems, improvement of crystallinity was intended in the aforementioned Patent Documents by increasing the molecular weight or by crosslinking in attempts to obtain a foam.
Meanwhile, a polyhydroxyalkanoate (hereinafter, referred to as P3HA) resin produced from a microorganism is disclosed in Japanese Unexamined Patent Application Publication No. 2000-319438 in connection with P3HA resin foamed particles, and a molded product thereof. In Japanese Unexamined Patent Application Publication No. 2000-319438, there is described a method of obtaining foamed particles having two melting points using poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (hereinafter, abbreviated as PHBH), which is a kind of P3HA, in a pressure tight container through using water as a dispersion medium, and isobutane as a foaming agent.
However, substantially, only foamed particles consisting of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) a lone were studied.